Hi guys
I have two of these, fine power amps
I use them as hifi poweramps, monoblocked with a Rotel pre amp.
I noticed last week, (and this may have been the case for a long while), that when playing piano music. with the gain on these anps at half, a minor sibilance, bell like distortion, and a like a "sucking of air* sound around piano music.
On the rare time I played a CD with some piano music at the beginning, i put it down to a bad recording.
Seems it does it with every stand alone, piano, harpsichord, etc very strange. And they are also very consistently at the same level of gain, distorting, slightly. I dont listen to piano concertos , but rarely, but its ruined that, im at a loss, have sprayed switch cleaner on the pots, that is all, no difference.
If i take the gain potentiometers on the A500,s very low, and turn the preamp up full, no problem. Also i put the gain on full, it sounds fine again, but because the gains up full, the noisefloor is higher, and so its a bit noisy.
I just wonder if anyone would have an idea, as i tried both amps, on both the same gain level, and both are the same, could it maybe be a fault of the potentiometers, or just poor design circuitry?. The preamp works ok, as i used it with another pro amp i have, made by Samson.
if you had Any ideas i would be grateful?
I have two of these, fine power amps
I use them as hifi poweramps, monoblocked with a Rotel pre amp.
I noticed last week, (and this may have been the case for a long while), that when playing piano music. with the gain on these anps at half, a minor sibilance, bell like distortion, and a like a "sucking of air* sound around piano music.
On the rare time I played a CD with some piano music at the beginning, i put it down to a bad recording.
Seems it does it with every stand alone, piano, harpsichord, etc very strange. And they are also very consistently at the same level of gain, distorting, slightly. I dont listen to piano concertos , but rarely, but its ruined that, im at a loss, have sprayed switch cleaner on the pots, that is all, no difference.
If i take the gain potentiometers on the A500,s very low, and turn the preamp up full, no problem. Also i put the gain on full, it sounds fine again, but because the gains up full, the noisefloor is higher, and so its a bit noisy.
I just wonder if anyone would have an idea, as i tried both amps, on both the same gain level, and both are the same, could it maybe be a fault of the potentiometers, or just poor design circuitry?. The preamp works ok, as i used it with another pro amp i have, made by Samson.
if you had Any ideas i would be grateful?
You didn't mention the age of the amp. I would surmise that problems like this are related to leaky capacitors that are likely in the signal path. Many times I see 1 uf polarized caps in the signal path of amplifiers when they really should be non-polar ones. Problems with capacitors generally occur after about 5 years.
my amps are just out their year long warranty by 2 months.
typical
Ive included an internal pic, of the Amp, its laid out like a quad
typicalIve included an internal pic, of the Amp, its laid out like a quad
An externally hosted image should be here but it was not working when we last tested it.
but as far as the decent sound, i think with components, thats where the similaritys endAnother possibility is TIM distortion or Transient Intermodulation Distortion. I found this on the web:
http://www.hometheaterhifi.com/volume_11_1/theta-dreadnaught-2-amplfiier-1-2004-part-2.html
Maybe it's time you build a Leach AMP. ~30 years old and still strong.
My dbx 4bx, when it was working had derivative processing which they dubbed impact restoration. It made LP's sound more like CD's and you could dial the amount of "attack" that could be heard. You could actually adjust the control to hear the hammers hitting the strings on a hammer dulcimer or a piano. I would occaionally take some amps that I serviced and hook iit up to my system and some couldn't take the processing. It may have actually sounded like what you are describing.
So, it could very well be a lousy design.
http://www.hometheaterhifi.com/volume_11_1/theta-dreadnaught-2-amplfiier-1-2004-part-2.html
Maybe it's time you build a Leach AMP. ~30 years old and still strong.
My dbx 4bx, when it was working had derivative processing which they dubbed impact restoration. It made LP's sound more like CD's and you could dial the amount of "attack" that could be heard. You could actually adjust the control to hear the hammers hitting the strings on a hammer dulcimer or a piano. I would occaionally take some amps that I serviced and hook iit up to my system and some couldn't take the processing. It may have actually sounded like what you are describing.
So, it could very well be a lousy design.
mikeks said:
I have actually worked on an otherwise identical amp (Alesis), so i know what i am talking about.
http://fileshare.eshop.bg/downloadsm/24051/Alesis_RA500.html
Hi Mikeks,
Alesis RA500 may look the same as Behringer A500 on the outside but not on the inside.
I'm pretty certain that Behringer A500 has "flying rail" topology with OP front end (I guess), I think similar to QSC USA 400.
On pics of A500 you can clearly see only two supply leads from transformer to each amp (no center tap) that confirms floating "flying rails" topology with AC coupled output.
Alesis RA500 may look the same as Behringer A500 on the outside but not on the inside.
I'm pretty certain that Behringer A500 has "flying rail" topology with OP front end (I guess), I think similar to QSC USA 400.
On pics of A500 you can clearly see only two supply leads from transformer to each amp (no center tap) that confirms floating "flying rails" topology with AC coupled output.
I had the same thing with some of my amplifiers. The problem was that the stand by curent (or how is it called in enlish) was too low. I had to adjust VBE a little bit more. I realized that it happend allso when the output transitors are not mached then it allso needs a little bit more "push"
I was very nervous when i heard that augfull piano sound! It can allso be heard with piccolo, organ instruments or any other "breathing" instruments.
I was very nervous when i heard that augfull piano sound! It can allso be heard with piccolo, organ instruments or any other "breathing" instruments.
Hi emperor,
I have not auditioned nor seen this amp in real but judging from opinions on the web there is mixed impressions. Seen that some has reported sound improvements by upping bias a bit.
On the other hand, you said sound is fine provided pots are positioned close to end positions, this speaks for something different than bias issues I think.
Just some thoughts....
With your volume pots at approx mid position they have the greatest source impedance into next coming stages.
Could it be that signal in the long screened leads to/from the potentiometers gets inductively affected by high current leads located to close?
If you run the amp non bridged does the same issue occur on both amps/channels?
I have not auditioned nor seen this amp in real but judging from opinions on the web there is mixed impressions. Seen that some has reported sound improvements by upping bias a bit.
On the other hand, you said sound is fine provided pots are positioned close to end positions, this speaks for something different than bias issues I think.
Just some thoughts....
With your volume pots at approx mid position they have the greatest source impedance into next coming stages.
Could it be that signal in the long screened leads to/from the potentiometers gets inductively affected by high current leads located to close?
If you run the amp non bridged does the same issue occur on both amps/channels?
Yes it does, and i have two of them, that do it in stereo mode or monobridged, which would indicate, poor design, or (I swear i would have noticed before , the piano), failing components.
I personally am a technological dunce,
and am asking for ideas, so i can get somone in the UK to fix it for me.A bigger phrase might be "quiescent current".
Definition: quiescent: being at rest; quiet; still; inactive or motionless from www.dictionary.com.
You can stick to "bias current", so that people don't have to pick up the dictionary. <G>.
Definition: quiescent: being at rest; quiet; still; inactive or motionless from www.dictionary.com.
You can stick to "bias current", so that people don't have to pick up the dictionary. <G>.
emperor said:
I have one A500 myself.
I noticed that with the A500's front control at max (zero attenuation) and its inputs shorted with a resistance (please excuse my bad english) the amp is completely silent from the speakers.
This means for me that the amp is silent in itself.
Nevertheless when I connect the preamp and the source I get some noise (maybe a ground loop ?).
By the way I have not understood well in which conditions you get the distortion.
I never get distortion with the A500 front control at max, just some noise.
I read some test report very positive about this unit, also in terms of distortion.
Thanks and regards,
beppe
Sorry for bumping this thread, but wanted to post the schematic of one of the Behringer A500 channels, it indeed is using flying rails as a topology for lack of a better description. I was expecting something like a standard Miller approach, maybe with some refinements (folded cascode etc.) so was somewhat dissapointed.
Cheers,
Sander.
Cheers,
Sander.
Attachments
There is a video on youtube showing distorsion problems if the A500 is NOT at max volume. So, keep it at max, and attenuate it at preamp.
If there are any A500 wizards out there, I could use some help. My amp is faulty on channel one. As soon as I turn the volume knob, it starts hissing in both speakers. At approx 5 sec intervals, both speakers produce a loud thump (lika a heartbeat). Speaker 1 begins thumping inward and speaker 2 outward.
If I keep the faulty channel at 0, the other channels works fine.
Best regards,
/Bo
If there are any A500 wizards out there, I could use some help. My amp is faulty on channel one. As soon as I turn the volume knob, it starts hissing in both speakers. At approx 5 sec intervals, both speakers produce a loud thump (lika a heartbeat). Speaker 1 begins thumping inward and speaker 2 outward.
If I keep the faulty channel at 0, the other channels works fine.
Best regards,
/Bo
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